The present invention relates to use of a stainless steel alloy, more particularly a ferritic-austenitic steel alloy for use in the shape of tubes intended for placement on the bottom of the sea, more particularly for exploitation of oil and gas deposits in the sea. The material according to the invention in such applications has to have good corrosion properties in aggressive chloride environments, good mechanical and physical properties, good fatigue properties as well as good compatibility with that types of fluids to be transported in the tubes. Tubes for this application are preferably manufactured as seamless hot extruded tubes.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
In order to extract oil from the bottom of the sea, holes are drilled down from the bottom of the sea into the oil deposit. On the bottom of the sea, a unit for controlling the flow and the continuous transport of the crude oil to those units where the crude oil will be handled and refined to useable products or semi-finished products, will be installed. The unit on the bottom of the sea has components such as valves which shall control the taking-up/pressure/flowrate etc., couplings and tubes which allow injection of chemicals down into the oil well. Often, methyl alcohol is injected for the purpose of preventing the crude oil from coagulating and causing unwanted stoppages in the production pipe.
The valves and the couplings are steered hydraulically or electrically from a platform, a production vessel or another unit at sea level of the sea or on land. A so-called umbilical connects the steering unit with the units on the bottom of the sea. That part of the umbilical that lies on the bottom of the sea, for instance between two undersea units at different extraction places, are called static umbilicals because these are subjected to movement on a relatively insignificant scale effected by the sea movements. That part of the umbilical, which is situated between the bottom of the sea and sea level is called dynamic umbilical and is to a great extend effected by the movements in the water and on sea level. Examples of such movements are currents in the water, movements of the platform/productionvessel and undulatory motions.
FIG. 1 shows a conventional umbilical tube 1 laid down on the bottom of the sea extending from a platform 2 which lies anchored on the level of the sea 3. In this umbilical, a number of tubes 4 are gathered together for hydraulic and electric control, together with a central tube for the injection of chemicals such as methyl alcohol. The umbilical could have different configurations dependent on the demands of service for the units on the bottom of the sea, but usually there is a bigger tube in the center for the injection of methyl alcohol with thinner tubes twisted around this. A cover 6 of plastic can be used to gather the umbilical tubes and make the entirety manageable for laying and installation.
The tubes 4, 5 in the umbilical tubes must be resistant to corrosion and have good mechanical properties. The tube material has to be resistant to corrosion in seawater, which surrounds the outer surface of the tubes. This property is considered to be the most important since seawater could be very corrosive to stainless steel. Furthermore, the material has to have a high corrosion resistance against the possibly corrosive solutions, which are be injected into the oil spring. The material has to be compatible with the hydraulic fluids, which are used for the hydraulic control without contaminating the fluid. Impurities in the hydraulic fluid could negatively influence the service function at the control unit on the bottom of the sea.
The mechanical properties of the material used to construct the umbilical tubes are very important. Since the depth can be considerable at the place for the oil extraction, the dynamic part of the umbilical becomes generally long and thereby heavy. This weight has to be carried by the platform or the floating production vessel. If the umbilical is made lighter, the available net lift could be used for other purposes than to lift the umbilical. In practice there are two different ways to reduce the weight of an umbilical with a determined configuration. A lighter material could be chosen or a material with the same density but higher yield point and tensile strength. By choosing a material with higher strength, tubes with thinner walls could be used and the total mass of the umbilical can be reduced. The deeper the sea at the extraction place, the longer the umbilical, and the more important the total weight per meter umbilical of the material.
Besides good corrosion properties and high strength, good fatigue strength properties are desirable. This is especially true for the dynamic part of the umbilical, which will be affected by the movements of the water and the floating unit to a large extent.
The general requirements for an umbilical can be summarized as follows:
Content of Fe: 35-55%
PRE (Cr+3.3Mo+16N): minimal 40
Yield point in tension Rp0.2 min=650 MPa.
Tensile strength Rm=800-1000 MPa.
Elongation A5 min 25%
Testing temperature acc. to ASTM G48A min 50xc2x0 C.
Testing temperature acc. to ASTM G48B min 35xc2x0 C.
Weldability
Good fatigue resistance
Until now, the most frequently used material for umbilicals has been a ferritic-austenitic stainless steel under the name Sandvik SAF 2507 and which is standardized under the designation UNS S32750. Until now this material has been well proven its ability to meet the corrosion resistance and strength requirements. This steel alloy is described in greater detail in the European Patent EP-A-220141, the disclosure of which is incorporated herein by reference in its entirety.
Accordingly, it is an object of the present invention to improve the performance of the above-mentioned components, including transport tubes, umbilicals and the like.
According to one aspect, there is provided a ferrite-austenitic alloy with a mixture defined as: C maximum 0.05%, Si maximum 0.8%, Mn 0.30-1.5%, Cr 28.0-30.0%, Ni 5.80-7.40%, Mo 2.00-2.50%, N 0.30-0.40%, Cu maximum 1.0%, W maximum 2.0%, S maximum 0.010%, 30-70% ferrite and the balance austenite for tubes filled with hydraulic fluid, as transport tubes for solutions for chemical injection, or another use in the application umbilicals.